This project is investigating the sites of thyroxine within the peptide portion of thyroglobulin. We have examined animal thyroglobulin by looking at its subunit structure, alterations by TSH and iodine deficiency, and peptides obtained with cyanogen bromide, trypsin, and Pronase. Only a few specific amino acids have been found by us as immediate neighbors of thyroxine, suggesting that the primary structure of thyroglobulin is important to the biosynthesis of thyroxine. We are applying the same approaches to thyroglobulin from normal and goitrous human glands, to see if abnormalities in thyroglobulin structure may be a basis for impaired thyroxine biosynthesis. Also, we have found changes in the sedimentation and amino acid composition of thyroglobulin from metastatic follicular carcinoma of the thyroid and are further exploring this possible relationship.